SSL_CTX_set1_curves to specify curves in Client Hello Supported Group Extension, not working

[Rohit Kaushal]

Hi,

I would like to ask if anyone here has actually been able to trim the EC
Supported Groups in the Client Hello with a TLS-ECDHE cipher using the APIs
described in the OpenSSL v1.0.2 manpage for SSL_CTX_set1_curves()
<https://www.openssl.org/docs/man1.0.2/man3/SSL_CTX_set1_curves.html>



My shared objects are built using OpenSSL v1.0.2t with FIPS Object Module
v2.0.5 . The TLS handshake pcap has always shown myCipher (
"ECDHE-RSA-AES128-GCM-SHA256") in the Client Hello correctly, accompanied
with 13 curves in the Supported Group Extension. However, enhancing SSL_CTX
myCtx to restrict the curves to just 3 (P-521:P384:P256) still show the
same 13 curves as in the pcap. The man page doesn't suggest nor do i see
any reason why FIPS should prevent this from working.


//existing working code

if((myCtx = SSL_CTX_new(SSL_METHOD *)TLSv1_2_method()) == NULL)

 return;

if((SSL_CTX_set_cipher_list(myCtx, myCipher) != 1))

 return;



//enhancement, not working

if(SSL_CTX_set1_curves(myCtx, "P-521:P-384:P-256", 3)) != 1))

 return;



The API returns 0 (suggesting no error), but pcap shows no change, i.e.
still shows the std. 13 curves.



Trying a code snippet involving SSL_CONF_cmd(myConfCtx, "-named-curve",
"P-256") suggested in this link
<https://superuser.com/questions/912311/how-to-force-a-server-to-use-a-specific-elliptic-curve-at-the-beginning-of-the-h>
,
available prior to SSL_CTX_set1_curves() introduction in v1.0.2, didn’t
help either.


Thank you for any guidance you can provide.

Rohit
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